A simplified, two-step preparative ultracentrifugation procedure was developed to isolate three density fractions of human plasma high density lipoproteins (HDL). In a study of fasting plasma from twelve normolipemic subjects, the concentrations of both the apolipoprotein and lipid constituents were variable in the two density fractions, HDL 2b (d 1.063-1.100) and HDL2a (d 1.100-1.130), and essentially constant in HDL 3 (d 1.130-1.210). Apolipoproteins A-I and A-II are major protein constituents of plasma HDL. Their daily transport into plasma from thoracic duct lymph was found to exceed their predicted synthesis. Thus, lymph HDL, which contains approximately 75% of lymph apoA-II, may to some extent represent recirculating HDL from the plasma. In a comparison of HDL cholesterol levels in cord blood from neonates from Cincinnati, OH and Leningrad, USSR, significant female-male differences existed in both populations while no significant Cincinnati-Leningrad differences were observed for either sex.